Estimating stem-scale mixing coefficients in low velocity flows

Stormwater ponds are SuDS devices intended to moderate the negative environmental impacts of stormwater run-off. A current, joint, research programme is investigating the effects of heterogeneous vegetation distributions in stormwater ponds and developing CFD techniques to simulate 3D solute transport processes in low velocity flows. The aim of the project is to generate a unique dataset that describes the influence of different types and configurations of vegetation on the pond’s fundamental flow – and treatment – characteristics. Better characterisation can then be used to better evaluate existing run-off treatment ponds that may be delivering sub-optimal levels of treatment. This paper presents results from an initial 1D laboratory study, with regular uniform emergent artificial vegetation, from which longitudinal dispersion coefficients have been evaluated over a range of target flow velocities. These have been integrated with stem-scale CFD mean velocity predictions to investigate the ability of a Chikwendu (1986) n-zone type approach to predict transverse dispersion coefficients over a scale suitable for inclusion in future 3D pond models. Assuming that the small stem-scale fluctuations in velocity form a repeating pattern at the patch scale, and that stem-scale transverse dispersion effects integrate, this approach has been successfully applied to predict mixing in a 2D system solely with parameters estimated from a 1D system.